A Metered Dose Inhaler (MDI) is a pharmaceutical delivery system that is commonly used to administer inhaled prescription drugs to treat a variety of conditions, including bronchospastic conditions such as asthma. MDI products contain aerosols which are generally solutions, primary emulsions, or suspensions of the active constituent along with propellant contained in a pressurized canister.
A conventional MDI consists of: 1) a canister, usually made of metal, approximately two inches high and one inch in diameter, 2) a metered dose valve and dispensing nozzle that is affixed to the top of the canister and is used to deliver a fixed quantity of the dose when the nozzle is depressed, 3) a plastic cap (the "mouthpiece") that is used to actuate the dispensing nozzle and to direct the dose into the patients lungs through the mouth; and 4) the contents of the canister, which usually consists of a mixture of an aerosol propellant and drug that is mixed or suspended in the propellant. The aerosol propellant acts as a carrier medium to deliver the drug. A propellant may be a single aerosol or a mixture of aerosols.
At room temperature, the aerosol propellant creates a saturated vapor pressure inside the canister. Upon actuation of the device, the internal pressure in the canister forces a metered quantity of liquid through the valve orifice and atomizes the actuated material. The pressure must be adequate to propel a metered quantity of the dose (propellant and medicinal drug) out of the canister each time the MDI is actuated, creating a plume or "puff". The expended dose passes out of the metering nozzle, through a small "transfer" channel in the plastic cap, and into a delivery channel that directs the dose into the patient's mouth. The patient inhales the administered dose while depressing and then releasing the cap (mouthpiece) of the MDI so as to actuate the metered dose delivery.
MDI products currently offer the benefits of bronchodilator, steroid, and other drug delivery with less systemic effects than would normally be observed following intravenous therapy. The MDI products also may be applied to the delivery of drugs such as Leukotriene antagonists as well as proteins and peptides.
The effective dose of drug delivered from a MDI product should be adequate and reproducible from actuation to actuation when treating a patient with asthma. For example, inappropriate and variable doses of bronchodilators during an acute asthmatic attack can have life-threatening consequences. Moreover, corticosteroids are often administered chronically to patients with asthma as a prophylactic measure and a high degree of dose-to-dose uniformity offers the greatest amount of protection. Two major determinants of the effective dose which a patient receives from a metered dose inhaler product are the dose delivered from the MDI, as well as the size of the particles inhaled into the pulmonary system.
Another consideration is that MDI pharmaceutical manufacturers typically label the packaging and/or the MDI with a "labeled dose", which is the maximum number of doses to be used. MDI products typically contain overfill to ensure good dose to dose reproducibility for the number of "labeled doses" of the product. For these reasons, it is essential that the device delivers an accurate and consistent dose from actuation to actuation from the first until the "labeled dose" is reached. It is also important that the particle size is maintained at a level acceptable to penetrate deeply into the pulmonary tree for maximum efficacy. The current manufacturing practice of overfilling MDI products permits actuation of the MDI well beyond the "labeled dose". When the "labeled dose" is exceeded, the dose strength could result in suboptimal therapy.
There are no easy, reliable ways currently available to the patient to determine when a canister's "labeled" contents have been consumed and that the patient has reached the "labeled dose". Typically patients will use metered dose inhaler products until the entire canister is exhausted. This could represent 25% or greater actuations beyond that labeled for this pharmaceutical product. Thus, a mechanism which would aid the patient in tracking the number of actuations which have been used for a given canister would aid the patient in determining when an MDI canister should be discarded and a new MDI canister used. The development of such a device would enhance the ability of a patient to comply or adhere to a prescribed dosing regimen. It is well appreciated that failure to adhere to medication dosage regimens can have a "profound influence on health care outcomes". (Levy; Pharm. Res. 12, No. 7: 943-944, 1995).
Any improvement in the treatment of asthma could result in reduced hospital stays and reductions in related health care costs. From a pharmacoeconomic standpoint, this would be highly desirable and third party insurers would likely provide reimbursement for the added cost of innovations which could reduce the cost of treating this disease in the hospital.
One approach to identifying each actuation of a MDI and to determining the number of actuations for a given canister is seen in U.S. Pat. No. 5,020,527 (Dessertine). However, the actuation of the MDI is identified by the mechanical operation of a lever each time the canister is depressed for delivery of a dose, and the number of actuations of the lever is counted. This approach is deficient because the requirement for a mechanical actuation of a switch presents the greater possibility of false actuations or failed actuations, thereby providing erroneous readings.
Accordingly, the invention is concerned with the solution of these and other problems presently encountered in the art.